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A study of drug use in Vancouver's Downtown Eastside concluded with 100 per cent of participants who used illicit opioids testing positive for fentanyl, raising concerns that higher opioid tolerance from the powerful synthetic drug could threaten the effectiveness of substitution treatment.

The five-month study, led by University of British Columbia psychiatry professor William Honer, involved 237 high-risk participants. Of those, about half used opioids, either prescribed (such as methadone and buprenorphine) or non-prescribed (such as illicit heroin). Severe mental-health issues also played a significant role: About half had psychosis and one-third had mood disorders, illnesses which increase the likelihood of using illicit drugs.

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Keith Ahamad, an addictions physician at St. Paul's Hospital and the B.C. Centre for Excellence in HIV/AIDS, viewed the findings differently.

"In this superhard to treat population, half the people aren't using [illicit] opioids, which is a major success," Dr. Ahamad said. He added that there is no evidence to suggest that higher opioid tolerance due to fentanyl may make substitution therapy less effective.

"It's true that all these medications have never been tested in the context of a drug market that is predominantly fentanyl, but it's still just an opioid – and the goal of these medications is to get rid of cravings, decrease [illicit] drug use and [maintain] tolerance, which decreases mortality for people who use drugs," he said. "If people stopped taking these medications, the mortality rate would skyrocket."